How could a theme 'go extinct'? Can you elaborate?
Functionally I really don't see how it could be a thing- once a pet exists, it exists- even if they're released, you can still find them by searching for Released pets, and released pets can always be reclaimed (never even mind purebreeds of the theme.)

The closest you can really get to 'extinct themes' are the ones that were released before tags were a thing (which still don't have tags, and are not present on the advanced search theme lists for that reason)
(but at least several of those have extensive player-driven documentation on which pets are of those themes and of their descendants, though I've never personally payed mind to these, since I joined after tags became a thing)

Unless you mean like, somehow nobody managed to find a zone pet of that theme in the entirety of the two years it was available (at which point people who did get their themes picked would probably pick it as their exclusive prize because of that fact) (but also seems... a highly ridiculous concept, considering how many people are playing during fests, how much, and how many perfumes get used)
(and ff aur are an exclusive theme rather than a zone theme- to have it so no pet of an exclusive theme existed, you'd need literally nobody to use any pets of whatever species on their team for the entirety of all three board rounds, nobody to nurture any of the species, nobody to make any via whatever seasonal creation method... It's just not gonna happen)

If you do just mean 'what if somehow all the pets of a theme including all the pbs were released', the answer is pretty much 'Nothing would happen'.
The theme would still be in the lists- that isn't dictated by members of the theme existing as owned pets, those are actively added in to the lists by Krinadon (It's part of the reason people get worked up a whole bunch about staring at advanced search in the leadup to fests; everyone antsy to see the new theme names ahead of time.)
And you could still advanced search the released pets.

Really if anything the likelihood is that someone would notice and seek to get reclaimed ones.

Link: https://www.sylestia.com/forums/?thread=87899&page=1#4 Author: Scathreoite Time Posted: 4/11/2020 at 7:07 AM If you do just mean 'what if somehow all the pets of a theme including all the pbs were released', the answer is pretty much 'Nothing would happen'. The theme would still be in the lists- that isn't dictated by members of the theme existing as owned pets, those are actively added in to the lists by Krinadon (It's part of the reason people get worked up a whole bunch about staring at advanced search in the leadup to fests; everyone antsy to see the new theme names ahead of time.) And you could still advanced search the released pets. Really if anything the likelihood is that someone would notice and seek to get reclaimed ones.

Also, with black friday, does that take into account 'released' pets, or just how many pets period on site, both released and on accounts? If it is only pets on accounts (ie not released) then any 'extinct theme' would likely be included in the black friday wheel.

If it does count released pets, then no theme could truly go 'extinct' except by those accounts that released them going inactive, which I think would again trigger the Black Friday thing? (I thought I remember something about it taking into account pets on active accounts? or is that another site?)

But as for the advanced search, since you can also search released pets, it wouldn't make sense to remove any themes, because technically those themes still exist, they are just released.

I think only a mass purge of released pets (and that might be a tad unpopular to do) would actually have the potential to make any theme go extinct. But I doubt they would remain extinct forever (again, due to my limited recall of the Black Friday wheel, it was for just that reasoning wasn't it? to prevent pets from going extinct due to being locked on inactive accounts or being released? as well as for pets that just didn't have high numbers to begin with)